bpf: remove tail_call and get_stackid helper declarations from bpf.h

commit afdb09c7 ("security: bpf: Add LSM hooks for bpf object related
syscall") included linux/bpf.h in linux/security.h. As a result, bpf
programs including bpf_helpers.h and some other header that ends up
pulling in also security.h, such as several examples under samples/bpf,
fail to compile because bpf_tail_call and bpf_get_stackid are now
"redefined as different kind of symbol".

>From bpf.h:

u64 bpf_tail_call(u64 ctx, u64 r2, u64 index, u64 r4, u64 r5);
u64 bpf_get_stackid(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);

Whereas in bpf_helpers.h they are:

static void (*bpf_tail_call)(void *ctx, void *map, int index);
static int (*bpf_get_stackid)(void *ctx, void *map, int flags);

Fix this by removing the unused declaration of bpf_tail_call and moving
the declaration of bpf_get_stackid in bpf_trace.c, which is the only
place where it's needed.

Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>